1. Field of the Invention
The present invention relates to segment type color liquid crystal display devices which display prescribed characters, graphics, etc., in color.
2. Description of the Prior Art
FIG. 1 is a plane view of a segment type liquid crystal display device 230 of the prior art. A plurality of segment electrodes 233 are arranged in the liquid crystal display device in a figure eight. Area A1 has a color filter, colored red for example, area A2 has a green color filter and areas A3 to A5 have blue color filters.
FIG. 2 is a cross section of the liquid crystal display device 230. The liquid crystal layer 237 is sandwiched between the glass substrates 231a and 231b transmissive to light and sealed by the sealant resin 238. A plurality of segment electrodes 233 are formed on the surface of the glass substrate 231a toward the liquid crystal layer 237, and also formed on this surface is an orientation film 234a. A plurality of common electrodes 232 are formed on the surface of the glass substrate 231b toward the liquid crystal layer 237, and also formed on this surface is an orientation film 234b. Polarizer plates 235a and 235b are provided on the surfaces of the glass substrates 231a and 231b opposite the liquid crystal layer 237. The red, green and blue color filters 236 mentioned above are formed on areas of the polarizer plates 235a including the segment electrodes 233 by a printing process.
FIG. 3 is a cross section of another configuration of the liquid crystal display device 230. Here, the color filters 236 are formed above the common electrodes 232 by an electrodeposition process or a printing process in order to eliminate color shifts due to parallax in the display area.
In this kind of normally black mode (negative display type) of liquid crystal display device 230, a voltage is applied between the common electrode 232 and the electrodes corresponding to the display areas to be displayed from among the segment electrodes 233 in the area A1, whereby light is allowed to pass through the liquid crystal layer corresponding to the segment electrodes on which the voltage is applied and a red color is displayed. In the other areas, as well, the same method is used to display green and blue.
In the liquid crystal display device disclosed in U.S. Pat. No. 3,840,695, color filters are disposed on the front and rear surfaces of the liquid crystal display element so only one color can be displayed in a single unit area.
Generally, liquid crystals are dependent on the light wavelength, so the blockage of light in the liquid crystal display device is not complete; i.e., light cannot be blocked over the entire visible wavelength band. Therefore, when performing positive display (black display color on a white background) in the liquid crystal display device 230, light passes through the liquid crystal layer 237 even when voltage is not applied on the liquid crystal layer 237, so the area A1 colored red in FIG. 1 can be distinguished from the other areas, thus degrading the display quality of the liquid crystal display device 230. Also, in the case of negative display (white display color on a black background), the blockage of light by the liquid crystal layer 237 is not complete, so when voltage is not being applied, the shape of the color filters 236 can be seen, thus degrading the display quality.
Further, since the display colors of the display areas are determined by the colors of the color filters at the process of production of the liquid crystal display device, the same display area cannot display different colors, thus detracting from the diversity of display and applicability.
Liquid crystal display devices utilizing a construction of arrangement, fine stripe-shaped color filters straight were disclosed in Japanese Laid-Open Patent Publications 60-260921, 61-239220 and 62-091917 to solve the above problems.
FIG. 4 is a cross section of a segment type color liquid crystal display device 301 utilizing stripe-shaped color filters. A color filter 305 is provided on nearly the entire surface of one surface of the glass substrate 304a, which is transmissive to light, by means of a printing process or an electrode position color process, and a plurality of segment electrodes 306 are formed on the color filter 305 corresponding to predetermined display areas. The color filter 305 is formed in a stripe shape parallel to the surface of the paper in FIG. 4 and a plurality of red, green and blue filters is arranged in a predetermined order in a direction perpendicular to the surface of the paper in FIG. 4. The segment electrodes 306 comprise an electrode for red, an electrode for green and an electrode for blue corresponding to the red filter, green filter and blue filter.
On one surface of the glass substrate 304b transmissive to light is formed a plurality of common electrodes 308 over the areas containing the display areas corresponding to the segment electrodes 306, and then an orientation film 307b is formed on the surface. The glass substrates 304a and 304b are disposed so that the surfaces face each other on which the orientation films 307a and 307b are formed, and a twisted nematic liquid crystal layer 309 is sandwiched between the glass substrates 304a and 304b and sealed with a sealant resin 310. The surfaces of the glass substrates 304a and 304b opposite the liquid crystal layer 309 are each provided with polarizer plates 311 and 312.
Here, the polarizer plates 311 and 312 are disposed to achieve parallel polarization, and so-called in 90.degree. twisted "normally black" display is performed; i.e., light is not allowed to pass through the liquid crystal display when voltage is not applied. By applying a voltage on one of the three electrodes which make up the segment electrodes 306, the desired color can be displayed at the display area corresponding to the segment electrodes 306. For example, when a voltage is applied on only the electrode for red, red is displayed in the display area, and when voltages are simultaneously applied on the electrode for red and the electrode for green, yellow can be displayed by the mixing of the red and green colors. Further, when voltages are simultaneously applied on the electrode for red, the electrode for green and the electrode for blue, white can be displayed.
The segment type color liquid crystal display device 301, which has the stripe shaped color filter 305 formed on the entire display area surface as mentioned above, can select so-called normally white display and normally black display by the arrangement of the polarizer plates. In the case of normally black display in construction of a single layer type liquid crystal display element, the incident light becomes elliptically polarized by the rotatory dispersion phenomenon in the liquid crystal, a leakage of transmitted light through the liquid crystal layer takes place even when voltage is not being applied, and a drop in the contrast of the image plane occurs.
The characteristics of the visual angle which is a measurement value by this inventor in the prior art segment type liquid crystal display device 1 are shown in FIG. 5. The solid lines l71 through l74 indicate the equal contrast curves when the contrast ratio CONT is 10, 20, 30, and 40 respectively. Therefore, the areas enclosed by each of the equal contrast curves l71 through l74 are the visual angle ranges when the contrast ratio CONT is 10, 20, 30 and 40.
As mentioned above, in the liquid crystal display device 301 wherein the color filter 305 is arranged in a stripe shape, when a color filter with a different kind of color is combined and a color different than the color of the color filter is displayed, there is a drop in the degree of mixing of the displayed color and in the clarity of the display color, and thus there was the problem of the display quality deteriorating.
Further, there was the problem of a leakage of light occurring and the contrast dropping in the normally black mode, because the liquid crystal display device 301 has a single layer structure.
In order to solve this kind of problem, a liquid crystal display device has been developed which has a two-layered construction with a liquid crystal display element for display and a liquid crystal element for compensation.
FIG. 6 is a cross section diagram of the two-layered liquid crystal display device 401. The liquid crystal display device 401 has a liquid crystal display element for display 402 and a liquid crystal display element for compensation 403, wherein the transparent common electrodes 405a, 405b, 405c and the segment electrodes 406a, 406b and 406c are formed on the pair of glass substrates 404a and 404b of the liquid crystal display element 402, and the color filter 407 is formed across the entire surface of the glass substrate 404a.
The color filter 407 is composed of minute stripe shaped filters, for example of red, green and blue, which are adjacent to one another and formed in large numbers across nearly the entire surface of the glass substrate 404a. The orientation films 408a and 408b are formed by coating nearly the entire surfaces of the glass substrates 404a and 404b, between which, for example, the twisted nematic type liquid crystal layer 409 is injected, and the periphery is sealed by the sealant 410.
On the other hand, the orientation films 412a and 412b are formed on the glass substrates 411a and 411b of the liquid crystal display element 403, between which also the twisted nematic type liquid crystal layer 413 is injected, and the periphery is sealed with the sealant 414. The polarizer plates 415a and 415b are arranged on the surfaces of the glass substrates 411a and 404b respectively which are on their mutually opposite sides.
One of the following is established for the liquid crystal display element 403: (1) a function which compensates the difference in the refractive indexes of the ordinary light and the extraordinary light in the liquid crystal display element 402, (2) a function which compensates the rotatory dispersion in the liquid crystal display element 402, or (3) a function which realizes the functions in (1) and (2) at the same time. This kind of liquid crystal display device 401 achieves normally black display when the polarizer plates 415a and 415b which are used are in the so-called cross polarization state, and the arrangement of the polarizer plates 415a and 415b is selected so that normally white display is achieved when they are in the parallel polarization state.
Here each of the areas of the color filter 407 which correspond to the segment electrodes 406a, 406b and 406c are the color filters 407R, 407G and 407B which selectively transmit red, green and blue light respectively, and a case will be assumed wherein the arrangement of the polarizer plates 415a and 415b is in the so-called normally black state. Then the display of 4 colors can be achieved corresponding to the display voltage application (ON)/non-application (OFF) state with respect to the segment electrodes 406a, 406b, and 406c, as shown in state 1, state 2, state 3 and state 4 respectively in the example 1 section of Table 1 below. Here the states 1, 2 and 3 indicate respectively the cases where the segment electrodes 406b and 406c are connected in common, or where the segment electrodes 406a and 406c are connected in common, or where the segment electrodes 406a and 406b are connected in common, and the display of 4 colors can be achieved in each example.
TABLE 1 ______________________________________ Segment Negative Type Electrodes (Normally Black) ON/OFF State Light Shutter Display R G B R G B color ______________________________________ Example 1 OFF OFF OFF Black 1 2 ON OFF OFF .smallcircle. Red 3 OFF ON ON .smallcircle. .smallcircle. Cyan 4 ON ON ON .smallcircle. .smallcircle. .smallcircle. White Example 1 OFF OFF OFF Black 2 2 OFF ON OFF .smallcircle. Green 3 ON OFF ON .smallcircle. .smallcircle. Magenta 4 ON ON ON .smallcircle. .smallcircle. .smallcircle. White Example 1 OFF OFF OFF Black 3 2 OFF OFF ON .smallcircle. Blue 3 ON ON OFF .smallcircle. .smallcircle. Yellow 4 ON ON ON .smallcircle. .smallcircle. .smallcircle. White ______________________________________
The display picture images of the above mentioned segment type color liquid crystal display devices 230, 301 and 401 are determined by the shapes of the segment electrodes 233, 306 and 406, and are inferior in the aspect of display diversity as they cannot be changed after the color liquid crystal display devices 230, 301 and 401 are manufactured.
In order to improve the diversity of the display, it is possible to display two kinds of display image planes in one color liquid crystal display device by stacking two of the liquid crystal devices for color display 402 shown in the above mentioned FIG. 6, forming the segment electrodes 406 to correspond respectively to different color filters, and forming each of them in different shapes. However, in this kind of liquid crystal display device, moire stripes are generated due to the stacking of two color filter. Moire stripes are the fine patterns resulting from the overlapping of two regular patterns and moire stripes seem to be wave patterns or clouing patterns corresponding to an angle of seeing.
Further, the color filter 407 is formed with stripe shaped filters of three colors (red filters, green filters and blue filters) which are of a width that cannot be distinguished by the naked eye, and it is possible to display one display image plane in different colors by forming the segment electrodes 406 to correspond to each filter. However, when a plurality of colors are combined and displayed, the degree of mixing and the clarity of color drop with respect to visual observation of the displayed color, resulting in the problem of deterioration in the display quality.
In the above mentioned liquid crystal display device 401, normally white display is realized instead of normally black display when put into a parallel polarization state by changing the arrangement of the polarizer plates 415a and 415b. In this case, selection of the transmission state for the red, green and blue transmitted light is achieved as shown in Table 2 below.
TABLE 2 ______________________________________ Segment Positive Type Electrodes (Normally White) ON/OFF State Light Shutter Display R G B R G B color ______________________________________ Example 1 OFF OFF OFF White 1 2 ON OFF OFF .smallcircle. Cyan 3 OFF ON ON .smallcircle. Magenta 4 ON ON ON .smallcircle. .smallcircle. Blue Example 1 OFF OFF OFF White 2 2 OFF ON OFF .smallcircle. Cyan 3 ON OFF ON .smallcircle. Yellow 4 ON ON ON .smallcircle. .smallcircle. Green Example 1 OFF OFF OFF White 3 2 OFF OFF ON .smallcircle. Magenta 3 ON ON OFF .smallcircle. Yellow 4 ON ON ON .smallcircle. .smallcircle. Red ______________________________________
In this kind of example of the prior art, only four colors are displayed with normally black display and normally white display for any of the compositions of the examples 1 through 3 in Table 1, resulting in the problem of a limitation being imposed on the display diversity as a color liquid crystal display device.
In the above mentioned liquid crystal display device 401, transparent electrodes are formed across nearly the entire surfaces of the glass substrates 411a and 411b of the liquid crystal display element 403 in order to switch the normally black display and the normally white display, and a technique for switching the application and non-application of the voltage to these transparent electrodes of the liquid crystal display element 403 is assumed. The case wherein the voltage is not apply corresponds to the above mentioned normally black state. In the case where transparent electrodes are formed on the liquid crystal display element 403 and the voltage is applied, the display state of the liquid crystal display device 401 becomes the so-called normally white state, and the transmission state of all red, green and blue transmitted light is selected as shown in Table 3 below.
TABLE 3 ______________________________________ Segment Positive Type Electrodes (Normally White) ON/OFF State Light Shutter Display R G B R G B color ______________________________________ Example 1 OFF OFF OFF White 1 2 ON OFF OFF .smallcircle. Cyan 3 OFF ON ON .smallcircle. .smallcircle. Red 4 ON ON ON .smallcircle. .smallcircle. .smallcircle. Black Example 1 OFF OFF OFF White 2 2 OFF ON OFF .smallcircle. Magenta 3 ON OFF ON .smallcircle. .smallcircle. Green 4 ON ON ON .smallcircle. .smallcircle. .smallcircle. Black Example 1 OFF OFF OFF White 3 2 OFF OFF ON .smallcircle. Yellow 3 ON ON OFF .smallcircle. .smallcircle. Blue 4 ON ON ON .smallcircle. .smallcircle. .smallcircle. Black ______________________________________
In this case, which corresponds to the switching from normally black display to normally white display, that is the background color is switched from black to white, for example as opposed to the case of state 1 in the example 1 section of Table 1, wherein display voltage is applied to only the segment electrode 406a while the display voltage to segment electrodes 406b and 406c is shut off, (R, G, B)= (ON, OFF, OFF) and the display color was red, the display color changes to cyan, which is the complementary color of red, regardless of the fact that the voltage application/non-application state of the segment electrodes 406a through 406c in the liquid crystal display element 402 does not change. The other display colors of light cyan and white in the example 1 section of Table 3 also change in the same way to red and black. The display colors of the other example sections 2 and 3 are also the same. Because of this, there is the problem of a limit being imposed on the diversity of display as a color liquid crystal display device.